The present invention concerns the preparation of wrought products possessing a fine recrystallized grain size from certain aluminum base alloys.
Aluminum base alloys, particularly those of the 3000 Series, as designated by the Aluminum Association, which have been formed into various wrought products such as sheet, are difficult to process to a fine enough recrystallized grain size to avoid development of the condition known as "orange peel" when the alloy is subjected to various forming operations. One solution to this problem which has been developed in the art is the application of high rates of heating to the anneal temperature of the alloy to reduce the grain size of the anneal product. Such methods, however, are low in productivity as adequate fast heating can only be conducted on a small scale with conventional equipment, or costly specialized equipment would be required which is difficult to justify economically in comparison with large scale batch annealing of large coils.
An additional problem relating to grain size exists which is peculiar to alloys such as Alloy 3003. Initially these alloys were cast into ingots by methods which produced slow cooling during and after solidification, and little trouble was experienced with grain size when such ingots were rolled and annealed. However, upon the introduction of the more efficient DC casting process, it was discovered that an extended high temperature homogenizing heat treatment of the ingot was necessary in order to avoid coarse grain formation upon later annealing. The long furnace time required for this homogenizing is costly and it creates a manufacturing bottleneck. The aluminum industry has done much to determine the cause of this effect, but it has not succeeded in altering the basic requirement. Even then, grain size problems frequently arise to plague the metal producer, especially when preferred orientations which cause "earing" during deep drawing must also be controlled. It has, therefore, long been an industry-wide objective to shorten or eliminate the homogenization and to either lessen sensitivity to heating rate or find an economical means of producing high heating rate.